Currently, almost all liquid fuels for ICEs and hydrogen for FCVs come from fossil fuels. Naturally, this situation cannot last forever, and sustainable alternatives will eventually be required. This article will review some of these alternatives.

Potential of sustainable fuels

The first and most obvious candidate for sustainable fuel production is biofuels. Estimates of total technical potential of biofuels vary widely, but central estimates for the long-term future are generally in the order of 100 EJ/year – roughly the rate of transportation energy consumption today. In the image below, L, M & H represent low, medium and high scenarios in two cases where biofuel production from food crops is allowed or not. The dotted lines show the range of scenarios for fuel demand.

The second candidate is synfuel from renewable electricity. Given the significant efficiency penalties of these processes, they will only be economical at very low electricity prices. Wind and solar start to create instances of such very low prices already at moderate market shares. In a renewable energy future, wind and solar might supply about 60% of electricity, after about about a third of the total generation potential is curtailed. If we assume a tripling of global electricity generation and 70% synfuel conversion efficiency, this excess wind and solar power can supply 56 EJ/year – about half of current transportation fuel demand.

Future high temperature nuclear reactors could also produce hydrogen through thermochemical process routes. There are no relevant bounds to the rate at which these fuels can be supplied, but these reactors will need to overcome a number of technical, economic and political hurdles.

Any marginal supply beyond these sustainable sources can be met by conventional fossil fuel pathways with CCS. We have enough resources and CO2 storage capacity for several centuries of such marginal supply.

It is therefore clear that the world has sufficient potential for long-term transportation fuel production. In the next sections, we’ll take a look at the economic aspects.

Biofuels

Fuels produced from cellulosic plant matter have a much smaller negative impact on the environment and food supply than first generation fuels like corn ethanol. The IEA thinks that these technologies have the potential to reach prices competitive with oil at $45-70/bbl.

Such next-gen biofuels also have much lower greenhouse gas emissions than regular gasoline (60-100% less). This will ensure competitive costs below $100/bbl even in an environment with high CO2 taxes. The breakeven electricity cost with electric drive is shown below for perspective:

Synfuel from wind and solar

Synfuel production offers an attractive method for productive utilization of wind and solar output that would otherwise have been curtailed. Plants can be built close to the highest concentration of wind/solar sources causing negative prices, thus requiring minimal electricity transmission costs. Easily exportable synfuels also offer a good solution to the challenge posed by the highly uneven renewable energy potential and population density around the world.

Naturally, it would be most efficient to charge battery electric vehicles (BEVs) with this low-cost electricity, but this will bring serious practical and economic problems. For solar-dominated systems, it will mean that the bulk of charging must happen in the few hours around noon. This will mean massive public charging infrastructure buildouts, expensive increases to distribution capacity, and substantial inconvenience for drivers. Prices in wind-dominated systems will fluctuate randomly and mostly over timescales that are longer than the daily cycle that is best suited to electric cars.

A strategically located synfuel plant can avoid these challenges. One potential issue is the low capital utilization rate, but growth and interest rates are likely to be very low by the time that this technology becomes relevant, making this of lesser concern. The cost estimates presented below assume a 3% discount rate and 30% capacity factor.

The following graph was created for hydrogen production from future PEM electrolysis. Hydrogen transport and storage costs are estimated from this old NREL report. Adjusted for inflation, short distance transport and storage of small quantities of hydrogen (e.g. to filling stations) would cost about $1/kg. Another $1/kg needs to be added for long-distance transport of large quantities of hydrogen (e.g. from a large plant to a distant population center). Long distance transport of small quantities of hydrogen (e.g. to an isolated filling station) costs $2/kg or more. An additional $0.5/kg was added for storage at the fueling station. Keep in mind that a kg of hydrogen has about the same energy as a gallon of gasoline where total refining, distribution and marketing costs are about $0.8/gal.

In practice, this means that a population center located close to a large concentration of wind/solar generators could get totally clean hydrogen fuel for a price equivalent below $100/bbl of oil, if the average electricity cost during the 2600 lowest cost hours of the year amounts to less than $20/MWh. As shown in the graph below (for California), such a situation is a real possibility, especially in scenarios with high solar PV market share.

A population center that is further away from the production site will pay about $1/kg ($45/bbl) more, and an isolated location another $1/kg on top of that. It is therefore clear that renewable hydrogen can be economical, but only under certain circumstances.

Compared to ICE cars, the maximum efficiency of FCVs will be slightly higher, although still below a BEV. The chart below shows the breakeven electricity price for BEVs to match FCVs over three different levels of BEV efficiency advantage.

One way to avoid the large transport and storage costs of hydrogen and to enable international trade is to transform it into a fuel that is a liquid at or near room temperature. Two main options will be reviewed here: liquid hydrocarbons (from Fischer-Tropsch) and Ammonia (from Haber-Bosch). Both processes should convert energy in hydrogen with an efficiency of about 80%. An additional hydrogen storage cost of $0.5/kg will be added to add a buffer between intermittent hydrogen supply from wind/solar and constant hydrogen consumption by the plant. Ammonia distribution costs are assumed to be double that of gasoline ($0.8/gal instead of $0.4/gal).

In this estimate, hydrocarbon synfuels are more economical than ammonia up to an effective CO2 price of about $60/ton. It should be noted that hydrocarbon synfuels will be produced using captured CO2, earning the plant a small credit because CO2 storage costs are avoided. However, it is likely that, even with this credit, longer-term CO2 prices make the ammonia option more economical. Ammonia can potentially be used in ICEs or fuel cells, although the fuel cell may be more costly and less efficient than a hydrogen fuel cell. The efficiency deficits of an ICE will therefore be used in the comparison. It is clear that, in terms of fuel costs, ammonia will only be able to compete in long-range applications where BEVs need to rely primarily on dedicated fast-charging stations.

Finally, it should be mentioned that such a large-scale rollout of electrolysis plants will also produce a large quantity of high purity oxygen as a byproduct. This oxygen can be used for simple oxyfuel CO2 capture with no energy penalty. 50 EJ/year of hydrogen production via electrolysis can produce enough oxygen to facilitate the highly economic capture about 2.5 Gt/year of CO2. One smart way of using this potential would be to combust fossil fuels on site to supply heat so that a high temperature electrolysis process can be used, reducing electricity demand per unit hydrogen by about a third. This will increase the technical potential of this route by 50%.

Synfuel from nuclear

A number of different pathways exist to produce hydrogen from heat and electricity produced by nuclear power. According to calculations in a fairly recent paper, hydrogen production costs could reach $2.5/kg in a high temperature reactor. As shown below, the most cost effective configuration produced both thermal (T) and electrical (E) energy to feed the hybrid sulfur hydrogen cycle.

For comparison, a cost of $2.5/kg corresponds to PEM electrolysis costs from electricity at $40/MWh reviewed in the previous section ($30/MWh for synfuels since no storage buffer will be required for hydrogen from a steady-state nuclear plant). In this price range, synfuels from nuclear may be just on the edge of competitiveness with electric drive in terms of fuel costs for long-distance transport applications.

Synfuel from fossil fuels with CCS

Promising methane reforming processes with inherent CO2 capture are under development. For example, a recent study on membrane assisted chemical looping reforming indicated that this process could produce hydrogen with CO2 capture at a similar cost as current conventional steam methane reforming, although the capital cost portion of the cost distribution was higher. The graph below shows the hydrogen production cost as a function of natural gas price using the cost assumptions in this paper.

Depending on natural gas price developments, this pathway can produce hydrogen at a similar or slightly lower price point than advanced nuclear discussed in the previous section. The process can also produce a pure stream of nitrogen, avoiding the air separation expense of ammonia production, bringing a moderate additional cost saving.

Discussion

Long-term biofuel developments are likely to keep ICE fuel costs competitive with electricity costs for fueling BEVs for most applications. In case biofuels substantially underperform expectations, ammonia or hydrocarbon synfuels produced from clean hydrogen will only be competitive on a fuel cost basis in long-range applications where a BEV would have to charge from dedicated fast charging infrastructure at higher electricity prices. Naturally, a long-range vehicle with an ICE or fuel cell will always be significantly cheaper than a long-range BEV (there is no need for a large battery pack).

Hydrogen produced from excess electricity during wind/solar peaks, thermal processes using nuclear heat, or fossil fuels with CCS will also remain competitive on a fuel cost basis for most transport applications. However, the transport and storage costs of hydrogen are significant, thereby limiting the locations to which hydrogen can be profitably distributed. Hydrogen can therefore not become a truly ubiquitous fuel source as oil is today.

Overall, a sufficient number of alternatives exist for clean fuel production in the long-term future to ensure a diverse transport energy mix. Electricity for BEVs, hydrogen for FCVs and biofuels/synfuels for ICEs will all have their place in the market. A diverse range of energy sources will ensure a reliable supply and a high degree of competition to keep prices low, thus benefiting consumers and general economic efficiency.

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Discussions

Nice article. A few comments:
I think the $.10/kW cost for night-time EV charging is likely high. If utilities don’t provide a compelling incentive for night-time charging, BEV owners will simply opt for evening fast charging, which will put a huge strain on the grid, and force a lot of expensive upgrades.

Regarding biofuel, it’s important to state that this option drives up our environmental footprint, so we should prefer batteries and synfuel where possible.

Regarding synfuel, my understanding is that starting from H2, making methane, methanol, or ammonia have similar efficiency (at least when ammonia is made at 100+ MW scale). But making syn-gasoline would be an extra step, thus would have lower efficiency.

Regarding ammonia versus H2: ammonia is much more suitable for seasonal energy storage and transportation. The result is that a certain fraction of the total H2 production is likely to come from dirty point-of-sale production (electrolysis, even on days when the grid mix includes significant fossil fuel backup, or distributed generation from fossil gas with no carbon capture).

A robust ammonia economy (say for long haul trucks) can also help to make H2 infrastructure (e.g. for cars) cleaner: hydrogen can be generated from tanked ammonia at the point of sale, as a way of supplying retailers that are not close to a hydrogen pipeline.

So now vast areas of farmland are turning to grass hay. Farmers are smarter than Californians and know wildfire isn’t the only environmental stewardship policy. And in Minnesota we at least intend to manage forests. But this is all expensive volunteer work, and we need some market mechanism, or when us old coots die off wolves will be roaming the streets of Minneapolis.

So, by long experience having never seen a biomass fire without light, I urge you to ask a young honest actual physicist how quantum mechanics works. We can “heat” silicon PVs and still not get electricity, so somebody somewhere knows something about quantum mechanics excitation and transitions. Personally, I get ill dealing with these modern social warriors. Solar energy can be stored into waste biomass derived fuels.

The categorization of synfuels and biofuels that you’re using and (especially) their methods of production aren’t clear to me. The way I think of it, there are three general approaches to synthetic fuels:

1) Approaches in which the energy in the output liquid derives from the input biomass. Production potential is ultimately gated by the photosynthetic productivity of the source biomass. There are three subcategories under this approach:

a) Alcohols. Production is via fermentation of sugars. The sugars can be derived from starches as part of the fermentation process, or they can be derived from cellulose in a separate treatment process.. Output liquid is usually ethanol, but butanol is a more desirable alternative if it can be managed.

b) Processed plant oils — aka biodiesel. Plant residue after oil extraction can be input for fermentation, boosting yield efficiency. But overall production potential is still gated by photosynthetic productivity.

c) Non-biological production. Flash pyrolysis of finely divided biomass is one option. It produces “bio-oil”, a witches brew of reactive molecules that will congeal into a gelatinous mass if left more than a day or so.

2) Approaches in which the biomass is used primarily as a source of reduced carbon.The energy in the produced liquid fuel derives partly from the biomass, and partly from the hydrogen used to convert the biomass to hydrocarbons. The liquid fuel yield per ton of biomass is roughly double, with this approach, compared to those in which biomass must supply all of the energy..

3) Approaches in which the source of carbon is CO2. Two molecules of H2 are needed to reduce CO2 to C (+ 2H20) for every H2 molecule in the synthesized hydrocarbon fuel. So this approach is the most energy intensive.

The energy needed to reduce CO2 is the strongest argument for using H2 directly rather than making synthetic hydrocarbons.

“For solar-dominated systems, it will mean that the bulk of charging must happen in the few hours around noon.” – this is only correct if a non existence of transportation grids is assumed. Otherwise with each 1000-1500km east-west extension of the grid, this time also extends by one hour.
The idea to start some synthesis path from waste biomass upwards to more energy dense and liquid materials using electricity might be possible, but I have no idea how, since that’s not my topic.
But I know that german chemcal plants use more and more biomass as starting point for different synthesis trees, because it is cheaper and saves the costs for the initial synthesis steps. Maybe something useful will come from that side sometimes.

we need some market mechanism, or when us old coots die off wolves will be roaming the streets of Minneapolis.

After the first winter they’ll be attracted to the plentiful supply of Somali-sicles.

by long experience having never seen a biomass fire without light, I urge you to ask a young honest actual physicist how quantum mechanics works. We can “heat” silicon PVs and still not get electricity, so somebody somewhere knows something about quantum mechanics excitation and transitions.

You can grasp this with classical thermo, if you know classical thermo.

Heat is the waste product of a PV cell, which is why it doesn’t work when you put heat in. The random thermal excitations don’t create electron-hole pairs; their entropy is too high (low temperature). Light and near-IR photons have far less entropy plus enough energy to knock electrons loose and create pairs.

You can create power from much lower-energy photons, even MHz photons, but they have to be low-entropy (coherent, like radio waves).

– this is only correct if a non existence of transportation grids is assumed.

I think you meant transmissiongrids.

Otherwise with each 1000-1500km east-west extension of the grid, this time also extends by one hour.

That’s true as far as it goes, but it reflects the implicit “transmission is free” fallacy that infects a lot of discussion about the economics of renewables.

Long distance transmission is most definitely not free. In fact, between the high capital cost of permitted lines and the low duty cycle of use, power imported from 1000 to 1500 km away ends up being some of the most expensive electricity found. And that’s for only a one hour shift.

High voltage DC transmission helps. I’m strongly in favor of building more of it. But it’s not a real game changer. It’s only one binary order of magnitude better than high voltage AC transmission.

But I know that german chemcal plants use more and more biomass as starting point for different synthesis trees, because it is cheaper and saves the costs for the initial synthesis steps. Maybe something useful will come from that side sometimes.

Yes, that’s the “biomass as a source of reduced carbon” path that I referred to in an earlier comment. If one has cheap biomass available, that’s the most productive way to use it.

I wanted to add a couple of points about hydrogen. Turns out hydrogen may not be as impractical as debunkers of the once-touted “hydrogen economy” have claimed.

Hydrogen has a deserved reputation for being hard (costly) to transport and store. It has a low volumetric energy density compared to natural gas, so it’s often thought that long distance pipeline distribution is infeasible. That’s a premature conclusion.

The metric for pipeline transport efficiency is energy lost to flow resistance in transport as a fraction of energy delivered. Flow resistance in a gas goes as the square of velocity, so efficiency can always be improved by transporting the same volume at a lower velocity in a larger pipeline. It’s an economic tradeoff between pipeline diameter and energy loss.

For a given pipeline diameter and energy delivery rate, hydrogen is indeed less efficient than natural gas. However, it only requires a doubling of pipeline cross section (41% increase in diameter) for hydrogen to achieve roughly the same efficiency metric as natural gas.

With current “trench and fill” methods of pipeline construction. a 41% increase in pipeline diameter is a significant economic penalty. However, if “trench and fill” is replaced by the environmentally friendlier alternative of tunnel boring and pipe jacking, a larger pipeline diameter can be turned into what programmers like to refer to as “a feature, not a bug”.

To become practical as an environmentally friendly approach to pipeline building, tunnel boring will need to achieve the order of magnitude improvements in cost per mile that Elon Musk is targeting for his Boring Company. I think it’s feasible. Assuming it is, the “feature” part of a network of deeply buried, large diameter pipelines is that the network provides integrated energy transport and storage. The total volume is so large that it can easily accommodate storage needs for diurnal and weather-related variations in supply. I haven’t run the numbers, but I suspect it could even cover seasonal variations.

The other point concerns mobile hydrogen storage. In specific circumstances, there happens to be a very attractive solution. It goes by the handle of “CcH2”: cryogenic compressed H2. It’s just compressed hydrogen gas, stored in a high pressure tank, but chilled to very low temperature. The tank is insulated, but because of the specific circumstances where this approach applies, super insulation is not required.

The specific circumstances are that soon after the tanks are filled, use of the hydrogen they contain must commence. The expansion of gas remaining in the tank as a portion is consumed keeps it cold and prevents excess pressure build-up.Consumption must continue at least until about half the hydrogen supply has been consumed. Otherwise, hydrogen may need to be vented to prevent excess pressure build-up as the gas warms.

CcH2 is covered, briefly, in a DOE hydrogen program review conducted in Washington DC in 2010. The significant fact is that it achieves both the highest volumetric and gravimetric energy of any mobile hydrogen storage technology known. It even beats liquid hydrogen in volumetric energy density, and in gravimetric energy density when the weight of cryogenic tanks for liquid hydrogen storage is included. And the energy cost of chilling to liquid nitrogen temperature is only a fraction of the energy cost of hydrogen liquefaction.

The CcH2 approach seems well suite to buses, trains, long-haul trucks, short to medium range electric aircraft, and perhaps even to shipping. So we just need a cheap source of hydrogen itself.

It’s possible that much less fuel will be needed for transportation 20 years from now than people expect. There’s a good chance — in my opinion — that electrification of transport will proceed much more quickly than what is usually projected. If it happens, though, it won’t be due to dramatic improvements in battery performance and cost. It will be due to electrification of roads.

There are two approaches to electrification of roads that are commonly considered. One is overhead catenaries, the other is inductive pickup.

Overhead catenaries are a proven technology for trains and urban trolleys. There are demonstration projects in Europe testing it for highway trucks. But it’s expensive and unwieldy. I don’t think it’s really practical for general use.

Inductive pickup is aesthetically appealing: “look, Ma, no wires”. But absent low cost room temperature superconductors, I don’t think it will ever be feasible. Capital cost and charging losses are both way too high.

There’s another alternative that nobody seems to have considered. That’s in-road DC power rails. One rail is the ground return rail. It presents no hazard to anyone touching it, because it’s literally grounded all along its length. It’s the power rail running parallel to the ground rail a few feet away that seems scary. Anyone thinking in terms of ordinary power rails will instantly reject the concept as utterly crazy! However, the power rail I have in mind is very far from ordinary.

The power supply rail in the system I’m thinking of is comprised of many isolated segments. Each segment has its own switch and controller. Any given segment is connected to power only when a vehicle that wants to draw from it is present on the segment. Furthermore, any vehicles drawing from the segment would continuously report to the controller precisely how much current they were drawing. The controller would continuously compare the sum of the values for authorized vehicles on the segment to what it was supplying. Any discrepancy would trigger an immediate disconnect. A daring person could stand barefoot straddling the ground and power rails, and be perfectly safe — at least from electrocution. They could well be hit by a fast-approaching vehicle, but that’s a different issue.

OK, so what is a transportation gid in my place is a transmission grid in your place – good to know.
Be aware that the cost difference between building a power line of low capacity, and one of high capacity at the same voltage is not really big, and that the maintenance difference is almost negible. Grids scale well, once there is a market for components with higher capacity.
The gid would trasport power (for PV) in the morning in one direction, and in the evening in the opposite direction, and overlayed with this the longer periods of transportation of one or the other direction for wind power.
This load factor is much higher than the load factor for grids which did exist for backup purposes only like the german grid in most part, which mainly had load factors round 0,1 in earlier times on the 400kVlevel usually.
Naturally a power line, transporting power from e.g. a remote run of the river hydropower station to a agglomoration, has a higher load factor. but such power lines are rare, at least here.

Costs of installation and maintenance, and the costs of maintaining the road surface are too high for this system. The overhead lines are the lowest cost option, and provide by far the highest amout of power, allowing to charge batteries ina haigh rate, which again allowes to eqip roads just partially (e.g. 30%) See here for deatils: http://www.bmvi.de/SharedDocs/DE/Anlage/MKS/studie-potentiale-hybridober...

If you want to talk really clever power transfer schemes, there’s one out of Japan which uses capacitive coupling from a buried conductor in the pavement to the steel belt in the tire. This would require low radio frequencies and resonant coupling to work. Skin effect would prevent outright electrocution of humans even if they were standing on the powered electrode, and the likelihood of someone having a wide enough stance to bridge two of them spaced at vehicle track width seems unlikely.

I have no idea what kind of efficiency this would have.

I personally favor a buried third rail inside a slot in the guard rail. This would be for fast-charging while in motion, not continuous down entire roadways. If you only need to cover 10% of the total length of the road to get 100% electrification, the costs go way down.

Algae is renewable, has no affect on the food channel and consumes CO2.
US taxpayers have spent over $2.5 billion on algae research. Exxon and others are working on algae fuels. It could be the next transportation fuel and the infrastructure is already in place. It has has already been proven in commercial airlines and in automobiles. Better lubricity and better gas mileage.

Costs of installation and maintenance, and the costs of maintaining the road surface are too high for this system.

And you know this, how?

The overhead lines are the lowest cost option, and provide by far the highest amout of power,

Same question. Your assertion about cost is contrary to what I found in one article in a trade magazine about “catenary-less power for light rail”. It quotes a city engineer saying the opposite — that overhead lines are much more expensive, disruptive, dangerous, and higher maintenance. He estimated the cost of installing overhead lines in an urban setting as $7 to $7.5 million per mile.

I’ve no idea where you get the idea that overhead lines provide “by far the highest amount of power”. That makes no physical sense. If anything, the opposite is more likely to be true, since ground level power supply lines can easily be much heavier than overhead lines suspended by catenaries.

Perhaps you’re basing your statements on material in the feasibility study that you reference. Unfortunately, I can’t read it. My German sucks, and Google translate doesn’t work for a downloaded PDF. But it may be comparing the overhead line option for trucks to in-road inductive pickup. In that case, the statements about cost and power would make sense. As I said previously, barring the advent of cheap ambient temperature superconductors, inductive pickup will never be practical.

Note that home charging includes the cost of a fully installed home charger. A fully installed cost of $700 returns a levelized cost of about $0.02/kWh over 30 years with a 5% discount rate.

I expect that fast charging will turn out to be quite expensive due to demand charges and costly infrastructure, so BEV owners will have a good incentive to charge at home at night for $0.08/kWh.

The LCA studies I’ve seen on cellulosic biofuels actually look quite good. I fully agree that environmental impacts should be reflected in the cost though, thus slightly reducing the cost benefit that biofuels have over synfuels from clean hydrogen.

The efficiency estimates of ammonia and hydrocabon synfuel production certainly have a significant error margin. The small difference between the costs of ammonia and liquid synfuels in this analysis is mostly due to the $0.4/gal greater distribution cost assumed for ammonia because it is not liquid at room temperature.

I’ve not looked at ammonia-to-hydrogen processes. Do you have an estimate of how much this will cost? It will have to be quite cheap if it will be worth adding an additional conversion step instead of just using ammonia as fuel directly.

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